Evaluation of biogas production and operational feasibility of anaerobic co-digestion of P6 and DAF sludge from a dairy factory

Abstract

Dairy factory wastewater treatment is commonly associated with high organic loading, energy consumption and sludge management challenges. High-strength side streams from dairy processing may contain recoverable organic energy and therefore have potential for anaerobic digestion and biogas production. This study evaluated the laboratory-scale anaerobic co-digestion feasibility of two high-strength dairy factory waste streams, P6 and dissolved air flotation (DAF) sludge, with particular focus on biogas production, pH control, chemical dosing demand, total solids/volatile solids (TS/VS) behaviour and recovery after feeding interruption.

The experiment was conducted using fifteen 1 L bottle anaerobic digesters operated under mesophilic conditions at approximately 34 °C. Four linked experimental stages were used. Stage 1 was a batch start-up test under different inoculum-to-substrate (I/S) conditions. Stage 2 was a transitional operation using a fixed P6:DAF ratio of 85:15. Stage 3 was a long-term semi-continuous co-digestion stage comparing P6:DAF ratios of 80:20, 85:15, 90:10 and 95:5. Stage 4 examined shutdown, re-feeding recovery and short-term response after a second shutdown. The main analytical indicators included cumulative and interval biogas production, pre- and post-dosing pH, NaOH addition, TS/VS, VS-based apparent conversion and limited gas composition results.

The results demonstrated that mixed P6 and DAF sludge could produce measurable biogas under anaerobic conditions. In Stage 1, the I/S = 1:14 group showed the highest blank-corrected cumulative biogas production and the strongest VS-added-based specific biogas response under the batch test conditions. Stage 2 helped reduce some initial differences among reactors and provided a more comparable basis for the later ratio comparison. In Stage 3, P6:DAF = 90:10 produced the highest net cumulative biogas increase, while P6:DAF = 80:20 showed strong performance and better within-group consistency. The P6:DAF = 95:5 group showed weaker net gas production and greater individual-digester variation. Stage 4 showed that residual gas production continued during shutdown and that gas production could recover after re-feeding, although recovery behaviour differed among groups and was affected by operational history and temperature disturbance in some digesters.

Preliminary application of the Stage 3 results to available Tatua site-flow data indicated an estimated raw biogas flow of approximately 582–926 m³ day⁻¹, an indicative active digester volume of approximately 11,300–16,500 m³, and an apparent VS-load reduction of approximately 4.46–5.61 t day⁻¹ under the mean-flow scenarios.

Because methane fraction was not continuously monitored, the gas production results were interpreted mainly as biogas production performance rather than strict methane yield. In addition, COD was not measured, and VS-based apparent conversion was treated as a relative indicator rather than a complete mass balance or biochemical conversion efficiency. Overall, the study indicates that anaerobic co-digestion of P6 and DAF sludge has technical potential for biogas production and resource recovery. However, practical implementation would require improved methane monitoring, more detailed stability indicators, longer-term operation, process optimisation and pilot-scale validation.